Remove mcnotify integration

It doesn't currently work anyway I believe.

.github/mcnotify.yml

@@ -1,13 +0,0 @@
-# Configuration for McNotify - https://github.com/kivy/mcnotify
-
-subscriptions:
-  - events:
-      # Events which trigger notifications
-      labeled: api-break
-    # Issue used for posting notifications
-    issue: 5560
-    # Comment to post on the notification thread. Optional placeholders: {label},
-    # {issue-author}, {issue-title} and {issue-url}
-    comment: >
-      @{issue-author} proposed the following change: "{issue-title}".
-      Please follow the changes or comment at {issue-url}.